The invention relates to a hydraulic motor vehicle servo brake comprising a pedal-actuated master cylinder with at least one master piston, wheel brake cylinders connected thereto by way of at least one brake circuit, a brake valve inserted between the pedal and the master piston, which brake valve upon actuation by the brake pedal feeds pressure medium supplied by a motor-driven hydraulic pump connected to a supply reservoir in a controlled way to the master piston and directly to a brake circuit where it applies a controlled pressure. A pressure accumulator is maintained in loaded condition by the pump and being of such capacity as to ensure that it supplies sufficient pressure during the pump's running-up phase after the motor is started. A control valve is connected to the pump's pressure side and provided with a closing member which normally closes a passage to a return conduit leading to the supply reservoir and is pressurized against the force of a closing member spring by the pressure of the pump and in the direction of the force of the closing member spring by the controlled pressure in such a way that it reduces the pressure of the pump when the controlled pressure is low and the pump is in operation. A control valve is pressurized by the controlled pressure, which valve is connected to the hydraulic conduit from the pump to the master cylinder and in the event of there being no controlled pressure connects the pump to the pressure accumulator and in the event of their being controlled pressure separates the pump from the pressure accumulator.
In a known motor vehicle brake of the described type (German patent application DE-OS No. 33 15 731) the closing member of the control valve is kept in idle position by a spring against the pressure of the pump until the pump pressure overcomes the prestress of the spring and, if necessary, the controlled pressure. Since the pump is switched on again and again between pedal actuations due to an electric switch provided at the pressure accumulator, in order to keep the pressure accumulator in loaded condition, the closing member spring must be of such strength that the closing member is kept in its idle position until the pressure accumulator is fully loaded. This means that even in case of very weak braking operations, such as occur for example when one has to wait at a traffic light, a considerable pressure, for example 50 bar, builds up behind the pump, although during such time only low pressure on the order of 5 to 7 bar is required.
The object of the present invention is to create a motor vehicle brake of the type referred to in which in case of a low pressure requirement during a braking operation only a comparatively low pressure builds up behind the pump, thus not only saving energy but also avoiding unnecessary pressure loads on the components involved.